Phase change Random Access Memory (PRAM) devices are well known. PRAM devices are described, for example, in U.S. application Ser. No. 10/827,687, filed Apr. 19, 2004, entitled Phase Change Memory Devices Having Phase Change Area In Porous Dielectric Layer And Methods For Manufacturing The Same, and U.S. application Ser. No. 10/827,639, filed Apr. 19, 2004, entitled Phase Changeable Memory Devices Including Carbon Nano Tubes And Methods For Forming The Same, both of which are assigned to the assignee of the present invention, the disclosures of which are incorporated herein by reference in their entireties as if set forth fully herein.
Generally, a PRAM is a semiconductor memory device that stores binary data by using an electrical property of a phase change layer pattern. The phase change layer pattern has a crystalline structure or an amorphous structure according to temperatures during the operation of the PRAM. The temperatures of the phase change layer pattern may be generated by Joule heat by the current applied to the pattern. The binary data may be defined as “0” when the phase change layer pattern has a crystalline structure having a low resistance, and the binary data may be defined as “1” when the phase change layer pattern has an amorphous structure having a high resistance.
A PRAM may consume relatively high current, particularly when the phase change layer pattern changes from the crystalline structure to the amorphous structure, because the phase change layer pattern may emit much more Joule heat in the amorphous structure compared to the crystalline structure. When the phase change layer pattern has a bar shape and contacts a contact node, it may be desirable to decrease a contact area between them, in order to reduce the current for the amorphous structure. Further, if the phase change layer pattern is formed in a contact hole, and contacts the contact node, it may be desirable for the PRAM to have a larger portion of the phase change layer pattern change the amorphous structure and have more Joule heat emit from a sidewall of the phase change layer pattern compared with a bar shape phase change layer.
U.S. Pat. No. 6,147,395 to Gilgen discloses a method for fabricating a small area of contact between electrodes. As stated in the Abstract of this patent, “[a]n electrode structure for use in a chalcogenide memory is disclosed. The electrode has a substantially Frusto-conical shape, and is preferably formed by undercut etching a polysilicon layer beneath an oxide pattern. With this structure, improved current densities through the chalcogenide material can be achieved”.
Moreover, U.S. Pat. No. 6,586,761 to Lowrey also describes a phase change material memory device. As stated in the Abstract of this patent, “[a] phase change material memory cell may be formed with singulated, cup-shaped phase change material. The interior of the cup-shaped phase change material may be filled with a thermal insulating material. As a result, heat losses upwardly through the phase change material may be reduced and adhesion problems between the phase change material and the rest of the device may likewise be reduced in some embodiments. In addition, a barrier layer may be provided between the upper electrode and the remainder of the device that may reduce species incorporation from the top electrode into the phase change material, in some embodiments. Chemical mechanical planarization may be utilized to define the phase change material reducing the effects of phase change material dry etching in some embodiments”.
It may be difficult to uniformly form a Frusto-conical shaped tip in the lower electrode using conventional semiconductor fabrication processes. This difficulty may arise because the Frusto-conical shaped tip may be formed by performing an undercut etching process in the lower electrode. It may be difficult to control an extent of etching the lower electrode during the undercut etching process.